I gots this question on a practice exam, and it's really got me thinking and wondering what other opinions may be. So here we go:
" The resistance of a wire ___________ as its temperature rises."
a) Increases
b) Decreases
c) Remains the same
d) Fluctuates
So the answer we've been told in class is B. The teacher quickly explains how OHMS law shows it decreases. This is because when you increase the amperage, the wire gets warmer, and with the voltage being the same, the resistance has to go down in order for ohms law to still make sense.
Ex: 120v with 14 amps will have 8.5 ohms of resistance, whereas 120v with 12 amps will have 10 ohms of resistance.
Now ohms law is something we're taught a lot about through the G3 course and it comes up all the time with electrical questions, but we're also taught to read the effing question properly when doing exams, and take careful note of every word, as it can lead to an incorrect answer by simply not reading it properly.
When I look back at the question, I see nothing about volts, or amperage listed. Or anything about formula's for ohms law. I see a very basic question about a temperature increase of a "wire".
So in electrical engineering and physics, it's quite unanimous that pretty much all "wire" increases it's resistance with a rise in temperature.
I look at the question and have to assume and exact same voltage and amperage is present through this wire. It seems anything could be causing the increase in temperature (such as a change in night time temp to daytime temp, the sun shining on it, a wire laying over a heater, etc), and in this theory, the wire is definitely going to increase in resistance.
I'm going with A as the MOST correct answer for what is written in the question. But the teacher only looks at it from the basic ohms law rules. The formula's eventually go way beyond our scope with relation to temperature and its affect on electricity, but what are your thoughts? I would hope if this question is on the actual TSSA exam, it's listed as A and not B, but it seems it could be interpreted as either one.
I would say when current flows thru a circuit it creates heat due to resistance.
In your example higher amps cause a decreased resistance. The 14A circuit would have less resistance and less heat.
Quote120v with 14 amps will have 8.5 ohms of resistance, whereas 120v with 12 amps will have 10 ohms of resistance.
QuoteThe resistance of a wire ___________ as its temperature rises.
The answer is, A. Increases
If the question said "The resistance of a wire _____ as its amperage increases", the answer would be, B. Decreases. Ohms law says that if the voltage is the same the resistance will decrease when the amperage is increased, but this does not mean the wire temperature increases. The opposite is true. With more amps we have lower resistance and less heat.
Exactly. When he was bringing it up to the class, everyone was in agreement and had no issue with the answer he was giving. With it being a seemingly basic G3 question, it had me thinking I was either crazy, or everyone else in the class needs more studying on electrical theory (including the teacher).
its talking about the copper wire directly in terms of temperature, excluding voltages and amperage. If you took the resistance of 2 identical wires and one was 80 degress and the other was 400 degrees, the wire that was 400 degrees would have more resistance.
Yup. I had a large roll of thin wire and tested the resistance with it sitting out in the sun. I then let it sit in the freezer and tested the resistance again. Sure enough, the resistance was less after it was frozen.
Increases as it's heated. I have the practice test here, and the right answer is increases.
Quote from: nash668 on July 07, 2015, 08:48:56 PM
Increases as it's heated. I have the practice test here, and the right answer is increases.
You had the same teacher and very likely the exact same practice exam as me, and he said it increases was the correct answer?